Production and recovery of trichloroethylene



United States Patent PRODUCTION AND RECOVERY OF TRICHLOROETHYLENE Charles A. Bordner, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application December 28, 1954, Serial No. 478,196

5 Claims. (Cl. 260-654) ZCHChCHCh 08(0H)g The trichloroethylene can be recovered from the reaction mixture by steam distillation.

The reaction described proceeds smoothly at reflux temperatures, giving almost the theoretical yield of trichloroethylene, and is safe and easy to carry out. Some times, however, it is necessary to recover pure trichloroethylene from crude mixtures which already contain a substantial proportion of the material. A typical mixture may contain around 15-40% of trichloroethylene, some perchloroethylene, some tetrachloroethane and a remainder of other chlorinated hydrocarbons. Such mixtures may be obtained as residues from distilling the steam distillate of the preceding paragraph.

Attempts have been made to adapt the reaction given above to the purification of these crude mixtures. by means of a process such as the following:

Lime Residue Steam Distillation Distillation Steam Distillate Trichloroethylene It has been found in practice that addition of lime slurry to mixtures containing trichloroethylene followed or accompanied by reflux or distillation may be a hazardous operation. This is especially true if mixture contains relatively small amounts of HCl donating chlorocarbons. Dangerous explosions frequently occur when the procedure is carried out, resulting in damage to the equipment utilized and in injury to personnel.

The cause of the explosions noted has been traced to the presence of small amounts of dichloroacetylene, ClCECCl, in the gases evolved from the reaction mixture. This compound, produced by reaction of trichloroethylene with the lime slurry, is an easily detonated liquid boiling at 32 C. under standard pressure. Its vapors are spontaneously flammable in air and constitute a serious explosion hazard when present in very small amounts. Tests have demonstrated that concentrations of dichloroacetylene above about 115 p. p. in. (parts per million) in the steam distillate of Equation 2, above, tend to develop explosive concentrations during subsequent operations.

It may be noted that really explosive concentrations of the type indicated are not generally found until refiuxing or steam distillation has been in progress for a relatively long time. In laboratory operations carried out for only three or four hours, p. p. m. of dichloroacetylene are unknown. In large scale distillations, however, prolonged for 1014 hours, the formation of explosive amounts of dichloroacetylene is observed relatively frequently.

An over-all object of the present invention is consequently provision of an improvement in known methods for manufacturing and recovering trichloroethylene. Another object is provision of a method for reducing the quantity of dichloroacetylene formed during the liming of mixtures of chlorinated hydrocarbons containing trichloroethylene. Another object of the invention is pro vision of a method for preventing the formation of more than about 115 p. p. m. of dichloroacetylene in the liquid phase during prolonged liming and distillation of mixtures of chlorinated hydrocarbons containing trichloroethylene. A still further object is improving the process for obtaining trichloroethylene by liming mixtures of chlorinated hydrocarbons to eliminate the development of explosive concentrations of dichloroacetylene therein.

The above-mentioned and still further objects are achieved in accordance with this invention by a process in which the pH (measured at room temperature) of the aqueous lime slurry reacted with the crude trichloroethylene mixture is not allowed to rise above about 12.0. It is found experimentally that best results are obtained with a pH of l1-1'1.5. Reducing the pH to still lower values may unduly lower the speed of the reaction. When short distillations are involved, the pH of the slurry may be as high as about 12.0.

The pH of the slurry is most conveniently kept at the safe level by maintaining, calcium chloride therein. The calcium chloride may be added directly to the slurry or formed therein by partial reaction with added hydrocan also be accomplished by recycling some of the aque-,

ous product to the slurry after the trichloroethylene has been stripped from it since this product naturally contains calcium chloride.

The quantity of calcium chloride added to or maintained' in the lime slurry is not sharply critical. Generally, however, a maximum of around 10-15% of calcium chloride by weight, based on the weight of the slurry utilized, will be sufficient to lower the pH to within the preferred pH 11-12 range. When the chloride is formed in the slurry as by the addition of hydrogen or ammonium chlorides, the quantity of the added compound should be sufficient to develop the requisite amount of calcium chloride and concomitant pH. It is actually unnecessary to weigh the amount of material added since changes in pH of the slurry may readily be followed on a pH meter.

A preferred large-scale embodiment of the invention is as follows: A lime slurry is made up and the pH thereof adjusted to between about 11.0 and 12.0 by additions of calcium chloride solution. As noted, this adjustment can be made by direct measurement of the pH as by a pH meter. If the slurry is to be diluted, it should be diluted before the pH measurement because addition of water to a lime slurry containing calcium chloride will raise its pH. After the pH of the slurry has been properly adjusted, crude trichloroethylene is added to the slurry and agitated therewith for about 1-2 hours under reflux conditions. The product is then separated from the lime slurry by distillation with steam, a process which may require up to 10 or 14 hours. If desired, part of the spent calcium chloride solution can be used to adjust the pH of fresh lime slurry. It will readily be appreciated that the process can be operated in either a batchwise or a continuous manner.

The invention has been described primarily with reference to a crude mixture intially containing some trichloroethylene. It is, however, not so narrowly restricted. It may, for example, be used in any process which generates trichloroethylene during the liming of chlorinated hydrocarbons. A specific instance of this usage is that of Equation 1 in which tetrachloroethane is limed. Normally this reaction develops sufficient CaClz internally, but lime slurries of a lowered pH may he used to enhance safety if it is so desired. The process may also be used to purify slightly contaminated trichloroethylene or material already employer] once in a degreasin'g operation. Other applications will be evident to those skilled in the art.

There follow some examples which illustrate the invention in more detail. In these examples all percentages are by weight and the pressures utilized in all runs were ambient unless otherwise noted.

EXAMPLE 1 This example shows the effect of pH on the production I of dichloroacetylene during laboratory-scale liming and distillation of trichloroethylene samples.

A series of runs was made by adding 540 g. of an aqueous slurry of commercial lime of approximately 20% solid content to commercial trichloroethylene and steam-distilling the mixture. In some cases, additives were used with the lime slurry to lower or raise its pH. The total time consumed in each run was not more than about 3 or 4 hours. Data and results are shown in the C. P. lime and pure, freshly-distilled trichloroethylene were used in this run.

This table shows that CaCla has a marked effect in Cal repressing the formation of clichloroacetylene during the liming of mixtures containing trichloroethylene.

EXAMPLE 2 A series of about fifteen large-scale runs was made on mixtures of chlorinated hydrocarbons containing initially about of trichlorethylene. A substantial excess of lime slurry was used in each case. The pH of the slurry utilized had been lowered to 11.3 by means of additions of calcium chloride. Reflux time in each run was about 2 hours and the time required for steam distillation was about 1014 hours. The concentration of dichloroacetylene in the steam distillates varied between about 8 and 88 p. p. m., well below the 115 p. p. m. noted above as dangerous.

Having described my invention I claim:

1. The method of inhibiting the formation of clichloroacetylene during the liming of chlorinated hydrocarbons in the presence of trichloroethylene which comprises maintaining the pH of the lime slurries utilized in the liming operations at a value of 11-12.

2. The method of claim 1 in which the pH is main tained at around 11-11.5.

3. The method of claim 1 in which the pH of the lime slurry is maintained at the value of 11-12 by the maintenance therein of calcium chloride.

4. In a process of liming chlorinated hydrocarbons in the presence of trichloroethylene, the step of supplying calcium chloride to the lime slurry utilized in amount sufficient to maintain the pH thereof at about 11l2.

5. The method of obtaining trichloroethylene which comprises steam-distilling a crude mixture of chlorinated hydrocarbons containing trichloroethylene with a lime References Cited in the file of this patent UNITED STATES PATENTS MacMillan Nov. 15, 1921 Maude et a1. May 27, 1952 

5. THE METHOD OF OBTAINING TRICHLOROETHYLENE WHICH COMPRISES STEAM-DISTILLING A CRUDE MIXTURE OF CHLORINATED HYDROCARBONS CONTAINING TRICHLOROETHYLENE WITH A LIME SLURRY HAVING A PH MAINTAINED AT ABOUT 11-12.0, CONDENSING THE DISTILLATE FROM THE STEAM DISTILLATION AND RECOVERING TRICHLOROETHYLENE THEREFROM, ADDING PART OF THE RESIDUE CONTAINING CALCIUM CHLORIDE TO A FRESH LIME SLURRY TO MAINTAIN THE PH THEREOF AT ABOUT 11-12.0 AND REACTING THE FRESH LIME SLURRY WITH A FRESH BATCH OF CRUDE TRICHLOROETHYLENE. 